Scanning phase measuring method and system for an object at a vision station

ABSTRACT

A method and system are provided including an optical head which moves relative to an object at a vision station to scan a projected pattern of imagable electromagnetic radiation across the surface of an object to be inspected at a relatively constant linear rate to generate an imagable electromagnetic radiation signal. In one embodiment, the electromagnetic radiation is light to develop dimensional information associated with the object. The optical head includes at least one projector which projects a grid of lines and an imaging subsystem which includes a trilinear array camera as a detector. The camera and the at least one projector are maintained in fixed relation to each other. Three linear detector elements of the array camera extend in a direction parallel with the grid of lines. The geometry of the optical head is arranged in such a way that each linear detector element picks up a different phase in the grid pattern. As the optical head is scanned across the surface of interest, the detector elements are continuously read out. Depth an each point on the surface is calculated from the intensity reading obtained from each of the detector elements that correspond to the same point on the surface. In this way, the phases of the pattern are calculated from the three intensity readings obtained for each point. In another embodiment, the imagable electromagnetic radiation is polarized and the response of the detector elements is polarization sensitive. The generated images are based on polarization for the surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to U.S. patent application entitled “OpticalMeasuring System” filed Jun. 17, 1994 and having U.S. Ser. No.08/262,130.

TECHNICAL FIELD

This invention relates to non-invasive measuring methods and systemsand, in particular, to scanning phase measuring methods and systems foran object at a vision station.

BACKGROUND ART

Height distribution of a surface can be obtained by projecting a lightstripe pattern onto the surface and then reimaging the light patternthat appears on the surface. A powerful technique for extracting thisinformation based on taking multiple images (3 or more) of the lightpattern that appears on the surface while shifting the position (phase)of the projected light stripe pattern is referred to as phase shiftinginterferometry as disclosed in U.S. Pat. Nos. 4,641,972 and 4,212,073.

The multiple images are usually taken using a CCD video camera with theimages being digitized and transferred to a computer where phase shiftanalysis, based on images being used as “buckets,” converts theinformation to a contour map of the surface.

The techniques used to obtain the multiple images are based on methodsthat keep the camera and viewed surface stationary with respect to eachother and moving the projected pattern.

A technique for capturing just one bucket image using a line scan camerais described in U.S. Pat. No. 4,965,665 but not enough information isavailable to do a phase calculation based on multiple buckets.

Other U.S. patents which show phase shifting include U.S. Pat. Nos.5,202,749 to Pfister; 4,794,550 to Greivenkamp, Jr.; 5,069,548 toBoehnlein; and 5,307,152 to Boehnlein et al.

U.S. Pat. Nos. 5,398,113 and 5,355,221 disclose white lightinterferometry systems which profile surfaces of objects.

In the above-noted application, an optical measuring system is disclosedwhich includes a light source, gratings, lenses, and camera. Amechanical translation device moves one of the gratings in a planeparallel to a reference surface to effect a phase shift of a projectedimage of the grating on the contoured surface to be measured. A secondmechanical translation device moves one of the lenses to effect a changein the contour interval. A first phase of the points on the contouredsurface is taken, via a four-bucket algorithm, at a first contourinterval. A second phase of the points is taken at a second contourinterval. A control system, including a computer, determines a coarsemeasurement using the difference between the first and second phases.The control system further determines a fine measurement using eitherthe first or second phase. The displacement or distance, relative to thereference plane, of each point is determined, via the control system,using the fine and coarse measurements.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and systemincluding an optical head for making an optical phase measurement of aviewed object by generating an image whose intensity as a function ofposition relative to the optical head and wherein the system isconfigured in a way which allows multiple images with different phaseinformation as the viewed object is moved in a direction perpendicularto the imaging system and these multiple images are used to calculate aphase image that is proportional to the optical phenomena that createsthe phase change.

Another object of the present invention is to provide a method andsystem including an optical head for making a phase measurement ofimagable electromagnetic radiation returned to a multi-line lineardetector array by setting up the optics in the optical head in a mannersuch that a different phase value is imaged onto each line of thedetector array such that each line of the detector array creates animage with a different optical phase value for the same point on theimaged object.

Yet still another object of the present invention is to provide a methodand system including an optical head for scanning the height of asurface wherein the optical head includes a light stripe projector andimaging system where the projected pattern does not move relative to theimaging system and the optical head is configured in a way which allowsmultiple images with different phase information as the surface is movedwith respect to the imaging system and these multiple images are used tocalculate a phase image that is proportional to the height of thescanned surface.

In carrying out the above objects and other objects of the presentinvention, a method is provided for high speed scanning phase measuringof an object at a vision station to develop physical informationassociated with the object. The method includes the steps of projectinga pattern of imagable electromagnetic radiation with at least oneprojector and moving the object relative to the at least one projectorat the vision station to scan the projected pattern of electromagneticradiation across a surface of the object to generate an imagableelectromagnetic radiation signal. The method also includes the steps ofreceiving the imagable electromagnetic radiation signal from the surfaceof the object with a detector having a plurality of separate detectorelements and maintaining the at least one projector and the detector infixed relation to each other. Finally, the method includes the steps ofmeasuring an amount of radiant energy in the received electromagneticradiation signal wherein the detector elements produce images havingdifferent phases of the same scanned surface based on the measurementand computing phase values and amplitude values for the different phasesfrom the images.

In one embodiment, preferably the physical information is dimensionalinformation and the imagable electromagnetic radiation is light.

In another embodiment, preferably the physical information ispolarization information, the imagable electromagnetic radiation ispolarized, a response of the detector elements is polarization-sensitiveand the images are based on polarization from the surface.

Further in carrying out the above objects and other objects of thepresent invention, a system is provided for carrying out the abovemethod steps.

The above objects and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a machine vision system including anoptical head for carrying out the method and system of the presentinvention;

FIG. 2 is a schematic view illustrating the details of a firstembodiment of the optical head of FIG. 1;

FIG. 3 is a schematic view illustrating a second embodiment of theoptical head of FIG. 1 wherein a grating is introduced on the imagingside to create an optical moire pattern; and

FIG. 4 is a schematic view illustrating another embodiment of theinvention wherein a pattern of polarized electromagnetic radiation isprojected.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 1, there is illustrated schematically a machinevision system, generally indicated at 10, including an optical head,generally indicated at 12, for carrying out the method of the presentinvention. The method and system 12 of the present invention areprovided for high speed, scanning phase measuring of an object 14 at avision station 16 to develop dimensional information such as heightinformation of a surface 18 of the object 14. The object 14 movesrelative to the optical head 12 as indicated by arrow 20.

In general, the invention relates to the non-invasive three-dimensionalmeasurement of surface contours using technology such as moiretechnology with a novel approach that allows continuous scanning of asurface. A more general adaptation of this approach allows themeasurement of other optical parameters via the same scanning approachbut with a different optical configuration.

The machine vision system 12 typically includes an image digitizer/framegrabber 22 electrically coupled to the optical head 12. The imagedigitizer/frame grabber 22 samples and digitizes the input images froman image source such as a camera contained within the optical head 12 asdescribed in detail herein below. The frame grabber 22 places each inputimage into a frame buffer having picture elements. Each of the pictureelements may consist of an 8-bit number representing the brightness ofthat spot in the image.

The system 10 also includes a system bus 26 which receives informationfrom the image digitizer/frame grabber 22 and passes the information onto the IBM compatible host computer such as a Pentium PC 28.

The system 10 may include input/output circuits 30 to allow the system10 to communicate with one or more external peripheral devices such as adrive 31 or robots, programmable controllers, etc. having one or morestages. The drive 31 provides relatively uniform and continuous movementbetween the object 14 and the head 12. The I/O circuits 30 may support athree axis stepper board (i.e. supports multiple axis control) or othermotion boards.

As illustrated in FIG. 2, a camera of the optical head 12 preferablyincludes a solid state image sensor such as a trilinear array camera 24.For example, the camera 24 may be the Kodak CCD chip model KLI-2103which has 3 rows of detector or sensing elements 25 each having 2098 CCDsensing elements per row. Each row is physically separated by a distanceequivalent to 8 pixel elements. The camera 24 was originally designedfor color scanning with a red, green, and blue color mask over eachelement, respectively. For the present invention, the masks are not usedbut rather are removed.

The system bus 26 may be either a PCI, an EISA, ISA or VL system bus orany other standard bus.

The image digitizer/frame grabber 22 may be a conventional three channelcolor frame grabber board such as that manufactured by ImagingTechnologies, or other frame grabber manufacturers. Alternatively, theimage digitizer/frame grabber 22 may comprise a vision processor boardsuch as made by Cognex.

The machine vision system 10 may be programmed at a mass storage unit 32to include programs for image processing and/or image analysis, asdescribed in greater detail hereinbelow.

A monitor 34 is also provided to display images.

Referring again to FIG. 2, generally, multiple images with differentphases are obtained by moving the surface 18 of the object 14 whilekeeping a pattern 36 projected by a light strip projector 38 and thecamera 24 stationary with respect to each other within the optical head12. The optical head 12 (i.e. when the system 10 is a scanning moiresystem) has no mechanical or optical mechanism that changes the positionof the projected pattern 36. To obtain multiple phase images, there isrelative movement between the optical head 12 and the measured surface18.

Although taking images with movement in any direction could result inthe ability to obtain phase shifts, there is only discussed herein twospecialized cases. The first case is movement of the object 14 in adirection 20 perpendicular to an optical axis of a lens 40 of the camera24 thereby creating a camera image. The second case is movement of theobject 14 in a direction parallel to the optical axis of the lens 40thereby creating a second camera image.

As with CCD linear array scanning, the object 14 is moved in thedirection 20 which is perpendicular to both the optical axis of thelinear array camera lens 40 and the line of pixels in the linear arraycamera 24. Thus, as the linear array camera 24 is read out line by line,the image of the object 14 moving past is created row by row. Using thetrilinear array camera 24 for scanning produces three images of thescanned surface 18 with each image being offset by a certain number ofrows. This offset is a function of the spacing between arrays and therate at which the image of the surface 18 is moved past the sensingelements 25.

The concept of scanning phase measuring of the present invention isanalogous to the color sensing by the above-noted color trilinear arrayexcept the color filters are not present and each of the three scanninglines measures a different phase of the projected light pattern insteadof the color.

In terms of phase shifting technology, each scanning line measures adifferent “bucket,” and a three “bucket” algorithm is used on thecomputer 28 for measuring the phase of the projected light pattern andthis phase is proportional to the surface height of the object beingscanned.

Before the phase is calculated from the readings at each of the scanninglines, the three scanned images are registered so that the phaseinformation from each of the three buckets is from the same point on thescanned surface. The registration correction and the calculation of thephase could be continuous if the electronics can accommodate this modeof operation.

As described above, three scanning lines are utilized. However, there isno reason that more scanning lines cannot be used to increase the numberof buckets used in the phase calculation or to average more than onescan line for a bucket. For example, if one had 16 scanning lines, thesum of lines 1 through 4 could be used for bucket 1, the sum of lines 5through 8 could be used for bucket 2, the sum of lines 9 through 12could be used for bucket 3, and the sum of lines 13 through 16 could beused for bucket 4.

Case 2 alluded to above would most likely use a CCD area array in theoptical head 1 but could use a linear array or single pointphotodetector. In this case, as the surface 18 is moved toward or awayfrom the optical head 12, images are taken as the phase of theprojection changes. The analysis would consist of correcting forregistration between images and then using the images to create thebuckets needed for the phase calculation. If the camera imagestelecentrically or nearly telecentrically, then registration would notbe required.

Systems that employ the Case 2 set-up have been described for use inwhite light interferometry systems as described in U.S. Pat. Nos.5,398,113 and 5,355,221 but not for a moire (light stripe) application.

Although a method is described above for making phase calculations basedon a moire (light stripe) system, the described technique could also beapplied to any optical base phenomena where the phase is changed betweenthe images created when moving the object 14 of interest relative to theoptical head 12. Techniques that can create this phase change includemoire interferometry, white light interferometry, standard monochromaticlight optical interferometry, ellipsometry, birefringence, andthermo-wave imaging.

The use of polarization to create an ellipsometer illustrates anotheroptical based phenomena where phase is changed between the imagescreated when moving the object 14 of interest relative to the opticalhead 12. The adaptation of this scanning phase measuring technique toellipsometry and birefringence measurement can be understood as anadaptation of a rotating-analyzer ellipsometer (as described at pp.410-413 of the book entitled “Ellipsometry and Polarized Light,” Azzamand Bashara). The rotating-analyzer ellipsometer projects polarizedlight onto a surface and the polarization of the reflected beam (ortransmitted beam depending on geometry) is determined by rotating ananalyzer (linear polarizer) in front of the receiving detector. Theradiation received at the detector varies as a sinusoidal function thatit twice the frequency of the rotating analyzer. The amplitude of thesignal is to the degree of linear polarization of the light received atthe analyzer and the phase defines the angle of polarization.

Using the scanning phase measuring technique of the present application,the rotating-analyzer would be replaced by three or more analyzers, eachof which would have a row of detector elements (scanning lines) behindit to image the received radiation at difference polarized phase values.The object to be measured would be moved past the fixed projector anddetector system on an optical head 12″ as shown in FIG. 4, whereinpolarized light would be projected (instead of a light stripe pattern asdescribed for a height measuring system). Each of the scanning linesmeasures a different phase of the sinusoidal polarization signal.

Items in FIG. 4 which have the same or similar structure and/or functionto the items in the prior figures have a double prime designation. Forexample:

-   -   Reference numeral 12″ designates an optical head of a scanning        phase measuring ellipsometer;    -   Reference numeral 14″ designates an object whose polarization        response will be measured;    -   Reference number 18″ designates a surface of the object 14″        whose polarization response will be measured when using a        projector 38″;    -   Reference numeral 20″ designates relative motion of the measured        object 14″;    -   Reference numeral 24″ designates a trilinear array camera having        analyzers 25″;    -   Reference numeral 36″ designates projected polarized light;    -   Reference numeral 38″ designates a polarized light projector for        a standard ellipsometer;    -   Reference numeral 40″ designates an imaging lens; and    -   Reference numeral 42″ designates a polarized light projector for        an ellipsometer in a transmission mode (birefringence measuring        system).

Reference numerals 60, 61 and 62 designate an analyzer system in frontof detector lines wherein 60 designates a linear polarizer parallel tothe linear array 24″, 61 designates a linear polarizer at 45 degrees tothe linear array 24″, and 62 designates a linear polarizer perpendicularto the linear array 24″.

The example shown in FIG. 4 uses a trilinear array camera 24″ with theanalyzers (linear polarizers) 25″ set at 0°, 45°, and 90° for the threescanning lines. In terms of phase shifting technology, each scanningline measures a different “bucket,” and a three “bucket” algorithm isused on the computer for measuring the phase and amplitude of the signalreceived by this scanning analyzer system.

Referring again to the first embodiment of the invention, the opticalhead 12 includes the light strip projector 28 and the camera includesthe imaging lens 40 for focusing the scanned surface onto the trilineararray 24. The scanned surface is translated past the optical head 12 inthe direction of the arrow 20. To eliminate perspective effects in bothprojection and imaging, the project and imaging system should be eithertelecentric or nearly telecentric. A nearly telecentric system iscreated by having the standoff from the optics being much larger thanthe measurement depth range.

For this discussion, the data from the first linear array in thedetector is called b1 (for bucket 1). Likewise, the second and thirdlinear arrays is called b2 and b3, respectively. The pitch of theprojected light pattern creates a phase difference of ½ a cycle betweenb1 and b3. For each linear array, let b1(i,j), b2(i,j) and b3(i,j)designate the light intensity measurement for each linear array with jindicating the pixel number and let j indicating the scan number. Forexample, b2(25,33) would be the intensity reading of the 25 pixel of thesecond linear array taken from the 33 scan.

The phase value which is proportional to depth is calculated within thecomputer 28 using the light intensity reading from the trilinear arrayas the object 14 is moving uniformly past the optical head 12. Thepreferred equation is:phase value(i,j)=arctan[{b1(i,j)−b2(ij+m)}/{b2(i,j+m)−b3(i,j+2m)}]where m is an integer that provides the required image shift to matchregistration between b1, b2 and b3.

In like fashion, the preferred equation for amplitude value is:amplitudevalue(i,j)=(((b1(i,j)−b2(i,j+m))²+(b2(i,j)−b3(i,j+2m))²)^(+ 1/2+)

In some instances, it is desirable to project from more than one angle.For example, projecting from each side of the camera can reduceocclusion problems. Projecting with patterns having different contourintervals (the change in depth for one phase cycle) can be used toeliminate ambiguity if the measurement range is more than one contourinterval.

Measurements with more than one projector by including a secondprojector 42 can be accomplished by cycling the part past the opticalhead and changing which of the projectors 38 or 42 is on for each cycle.Or, one of the illuminating projectors 38 or 42 can be changed for eachscan of the array. For example, assuming two projectors, when j is even,the first projector 38 would be on and when j is odd, the secondprojector 42 would be on. For calculations to work out properly for thisalternating system, then m, the integer shift value, must be even. Thus,using this alternating approach, phase value image for the firstprojector 38 would be: phase value (i,2j) where j=0,1,2, . . . ; andphase value image for the second projector 42 would be: phase value(i,2j+1) where j=0,1,2, . . . .

If it is desirable to increase the pitch of the imaged grating pattern,a second grating 44 can be added to the imaging side as illustrated inFIG. 3. In some instances, it is desirable to include an imaging lensbetween the grating 44 and the array 24. The parts shown in FIG. 3 whichhave the same or similar functions to the parts of FIG. 2 have the samereference numeral but a prime designation.

The beat effect between the two grating patterns is the optical moireeffect and will increase the pitch imaged onto the detector. This can bedesirable when one wants to use a pitch finer than can be resolved bythe detector. That is, the primary pitch is less than the width of apixel.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention as defined by the following claims.

1. A method for high speed, scanning phase measuring of an object at avision station to develop physical information associated with theobject, the method comprising the steps of: projecting a pattern ofimagable electromagnetic radiation with at least one projector; movingthe object relative to the at least one projector at a substantiallyconstant velocity at the vision station so as to scan the projectedpattern of electromagnetic radiation across a surface of the object togenerate an imagable electromagnetic radiation signal; receiving theimagable electromagnetic radiation signal from the surface of the objectwith a detector having a plurality of separate detector elements whichare substantially uniformly spaced; maintaining the at least oneprojector and the pattern of imagable electromagnetic radiation and thedetector in a substantially fixed relation to each other; measuring anamount of radiant energy in the received electromagnetic radiationsignal with the detector wherein each of the detector elements producean image having a different phase of the same scanned surface based onthe measurement; and computing phase values and amplitude values for thedifferent phases from the multiple images.
 2. The method as claimed inclaim 1 wherein the physical information is dimensional information andthe imagable electromagnetic radiation is light.
 3. The method asclaimed in claim 2 wherein the detector has an optical axis and whereinthe step of moving is performed in a direction substantially parallel tothe optical axis and wherein the projected pattern of light is a stripeof lines.
 4. The method as claimed in claim 2 further comprising thestep of determining height of the surface of the object based on thephase and amplitude values.
 5. The method as claimed in claim 1 whereinthe physical information is polarization information, the imagableelectromagnetic radiation is polarized, a response of the detectorelements is polarization sensitive and wherein the images are based onpolarization from the surface.
 6. The method as claimed in claim 1wherein the plurality of detector elements are uniformly spaced andwherein the step of moving is performed uniformly and continuously. 7.The method as claimed in claim 1 wherein the step of computing includesthe step of registering the images.
 8. The method as claimed in claim 1wherein the detector elements are elongated in a direction parallel to adetector axis of the detector, and wherein the detector also has anoptical axis and wherein the step of moving is performed in a directionsubstantially perpendicular to the detector and optical axes.
 9. Themethod as claimed in claim 8 wherein the detector is a multi-lineararray camera.
 10. The method as claimed in claim 8 wherein each detectorelement is a row of CCD sensing elements extending substantiallyparallel to the detector axis and wherein the step of moving isperformed in a direction substantially perpendicular to the rows of theCCD sensing elements.
 11. The method as claimed in claim 1 wherein thestep of projecting is performed with two projectors.
 12. The method asclaimed in claim 11 wherein the step of moving includes the step ofcycling the object relative to the two projectors wherein the twoprojectors alternately project the pattern of imagable electromagneticradiation.
 13. The method as claimed in claim 11 wherein the twoprojectors alternately project the pattern of imagable electromagneticradiation during consecutive scans of the projected pattern of imagableelectromagnetic radiation.
 14. A system for high speed, scanning phasemeasuring of an object at a vision station to develop physicalinformation associated with the object, the system including: at leastone projector for projecting a pattern of imagable electromagneticradiation; means for moving the object relative to the at least oneprojector at the vision station at a substantially constant velocity soas to scan the projected pattern of imagable electromagnetic radiationacross a surface of the object to generate an imagable electromagneticradiation signal; a detector for receiving the imagable electromagneticradiation signal from the surface of the object and having a pluralityof separate detector elements which are substantially uniformly spacedfor measuring an amount of radiant energy in the imagableelectromagnetic radiation signal wherein each of the detector elementsproduces an image having a different phase of the same scanned surfacebased on the measurement; means for maintaining the at least oneprojector and the pattern of imagable electromagnetic radiation and thedetector in a substantially fixed relation to each other; and means forcomputing phase values and amplitude values for the different phasesfrom the images.
 15. The method system as claimed in claim 14 whereinthe physical information is dimensional information and the imagableelectromagnetic radiation is light.
 16. The system as claimed in claim15 wherein the detector has an optical component for receiving thereflected light signal, the optical component having an optical axis andwherein the means for moving moves the object relative to the at leastone projector in a direction substantially parallel to the optical axisand wherein the projected pattern of light is a stripe of lines.
 17. Thesystem as claimed in claim 15 further comprising means for determiningheight of the surface of the object based on the phase and amplitudevalues.
 18. The method system as claimed in claim 14 wherein thephysical information is polarization information, the imagableelectromagnetic radiation is polarized, a response of the detectorelements is polarization sensitive and wherein the images are based onpolarization from the surface.
 19. The system as claimed in claim 14wherein the plurality of detector elements are uniformly spaced andwherein the means for moving moves the object relative to the at leastone projector uniformly and continuously.
 20. The system as claimed inclaim 14 wherein the means for computing includes means for registeringthe images.
 21. The system as claimed in claim 14 wherein the detectorelements are elongated in a direction parallel to a detector axis of thedetector and wherein the detector also has an optical component havingan optical axis and wherein the means for moving moves the objectrelative to the at least one projector in a direction substantiallyperpendicular to the detector and optical axes.
 22. The system asclaimed in claim 21 wherein the detector is a multi-linear array camera.23. The system as claimed in claim 21 wherein each detector element is arow of CCD sensing elements extending substantially parallel to thedetector axis and wherein the means for moving moves the object relativeto the detector in a direction substantially perpendicular to the rowsof the CCD sensing elements.
 24. The system as claimed in claim 14further comprising two projectors, the two projectors projecting thepattern of imagable electromagnetic radiation.
 25. The system as claimedin claim 24 wherein the means for moving cycles the object relative tothe two projectors wherein the two projectors alternately project thepattern of imagable electromagnetic radiation during consecutive cycles.26. The system as claimed in claim 24 wherein imagable the twoprojectors alternately project the pattern of electromagnetic radiationduring consecutive scans of the projected pattern of imagableelectromagnetic radiation.
 27. The system as claimed in claim 14 whereinthe at least one projector and the detector at least partially define anoptical head.
 28. The method as claimed in claim 2 wherein the detectorhas an optical axis and wherein the step of moving is performed in adirection substantially perpendicular to the optical axis and whereinthe projected pattern of light is a stripe of lines.
 29. The system asclaimed in claim 15 wherein the detector has an optical component forreceiving the reflected light signal, the optical component having anoptical axis and wherein the means for moving moves the object relativeto the at least one projector in a direction substantially perpendicularto the optical axis and wherein the projected pattern of light is astripe of lines.